This invention concerns improvements made to thermal treatment units used during the manufacture of containers, such as bottles made of polyethyleneterephthalate (PET) , and capable of withstanding, during use and without appreciable deformation, relatively severe thermal conditions encountered during procedures such as filling the containers with a hot liquid or the pasteurization of their contents.
According to commonly assigned U.S. Pat. No. 5,229,042 a process suitable for the manufacture of containers of this kind consists of the following steps:
a) heating to a temperature at least equal to the PET-softening temperature the body only (excluding the neck) of a container preform made of noncrystalline PET; during this step, the neck of the container is already shaped and sized to its final dimensions;
b) molding the body of the heated preform so as to produce an intermediate container in which the dimensions of the body are approximately 20% in height and from 0 to 30%, measured transversely, greater than the dimensions of the final container to be produced, while the mold-shaped walls of the container are cooled to a temperature of approximately 5.degree. to 400.degree. C.;
c) next, heating the body of the intermediate container to a temperature of from 160.degree. to 240.degree. C. for about 1 to 5 minutes; and, during all or part of this body-heating procedure, also heating the neck under the temperature and time conditions required for the crystallization of the PET in the neck only, before undertaking relatively slow cooling of the neck; and
d) finally, molding once again the heated contracted intermediate body produced at the end of the preceding step to its final shape and size, for a period of approximately 2 to 6 seconds.
To implement this process, the aforementioned patent also proposes container-manufacturing equipment incorporating, in particular, a thermal treatment unit capable of performing step c) of the process. In one special form corresponding to an implementation of the process that is preferred because of the simplicity of design underlying it, the thermal treatment unit comprises:
1) a heating device used to heat the entire intermediate container, both body and neck, the body being heated to a temperature of approximately 160.degree. to 240.degree. C. to provide for the "recovery" and increased crystallinity of the PET in the body, and the neck being heated under time and temperature conditions suitable for crystallizing the PET of the neck alone;
2) cooling means configured so as to lower the temperature of the entirety of the container, both neck and body, the neck being cooled relatively slowly so as to crystallize the PET in the neck; and
3) potentially, but preferably incorporated into the thermal treatment unit, another heating device configured to heat the body of the container before the final molding step specified under stage d) of the process.
The main advantage obtained using the thermal treatment unit in the manufacturing equipment proposed in the U.S. '042 patent consisted in combining two steps, i.e., spherulitic crystallization of the PET in the neck alone and thermal treatment of the body of the container, which had heretofore been carried out separately in a crystallization oven and in an oven termed a "recovery" oven, respectively. By the word "recovery" is meant the relaxation of the internal stresses induced in the PET during the two-way stretching operation (step b) of the process which produces the intermediate container, the material being allowed to freely undergo deformation, this recovery step producing a container having a contracted, irregular body.
However, the practical utilization of a thermal treatment unit of this type meets with various difficulties which call into question the simple end-to-end juxtaposition of the conventional crystallization and "recovery" ovens.
The main difficulty lies in the search for optimization of the energy consumption of the unit, while also allowing for adherence to the accurate performance of the steps of the process being implemented. In fact, in order to be optimally used and to avoid energy wastage, the thermal energy generated in the ovens by electric heating requires that the containers travel through the oven, while being arranged as closely as possible to each other, so that all of the radiation is intercepted by the material to be treated. During treatment, the material becomes significantly deformed, as indicated above. The intermediate container placed in the "recovery" oven is oversized as compared to the final container (e.g., an intermediate container having a diameter of approximately 110 mm as compared with a final 1.5 liter container having a diameter of 90 mm), while the container emerging from the recovery oven is appreciably contracted and undersized (diameter of about 80 mm in the example under consideration). Accordingly, the choice of a spacing distance between successive containers which is compatible with the large diameter of the intermediate containers leads to considerable loss of thermal energy, and thus to excessive electrical consumption, when weighed against the result obtained during the succeeding steps involving the contracted containers, which are thus spaced unnecessarily far apart.
Another problem concerns the conditions under which the crystallization of the necks of the containers takes place. When crystallization is carried out on the necks of the preforms during an initial step, the preforms can be juxtaposed in succession in such a way that the thermal output of crystallization heating proves satisfactory. On the other hand, in the aforementioned process the crystallization of the material in the neck is carried out on a container whose body diameter, which is greater than that of the neck, leads to a significant spacing separation of the necks of successive containers, which impairs good thermal output. As an example, the center-to-center distance between the necks, which is 65 mm for the preforms, increases to 140 mm for the containers. The result is a significant decrease of thermal output and an increased length of the crystallization oven.
Still in the area of energy yield, it must be noted that the ovens used heretofore were equipped with heating tubes arranged in rows extending over the height of the bodies of the containers to be heated and which emitted short-wavelength infrared radiation, and with heating resistors, so that the bodies of the containers which passed through the oven were heated mainly, not directly by the infrared radiation, but indirectly by the heated atmosphere of the oven (convection heating). As a result, the energy yield of this type of oven was mediocre and electrical consumption high.
Furthermore, the correct implementation of the various steps of the container-heating process required observance, in particular, of the material-treatment times, as regards both heating and cooling. While proper mastery of the heating conditions, in particular by reducing losses to the fullest extent possible so that the highest possible fraction of the thermal energy generated actually reaches the material to be treated, makes it possible to minimize the heating time, cooling, or temperature reduction, must, on the other hand, occur under completely controlled time conditions (e.g., approximately 20 to 25 seconds) , so as to preserve actual uniform crystallization in the material. Now, the search for ever-increasing manufacturing outputs (e.g., 6000 containers/hour) calls for high travel speeds for the containers in the equipment, and thus a lengthening of the distances traveled in the cooling stations, so as to conform to the required cooling times. The result is a concomitant lengthening of the equipment, thereby raising installation costs.
Another problem lies in the need for the effective thermal protection of the moving mechanism, which must be able to function without problems in atmospheres of varying temperatures (between 100.degree. and 200.degree. C.) and under very high temperatures (approximately 200.degree. C.) in the areas of intense heating.